December 20, 2013

Watch Out Buggles, Black Hole Also Killed The Radio Star

Astronomers have discovered a new type of exploding star that exhausts all its energy before collapsing into a black hole.

Scientists believed all gamma-ray bursts were followed by a radio afterglow, so Australian astronomers of the Centre for All-sky Astrophysics (CAASTRO) at Curtin University and the University of Sydney decided to set out to prove this theory. Instead they discovered a new population of exploding stars that switched off their radio transmissions by emitting one last strong beam of highly energetic radiation before they died.

“After studying an ultra-sensitive image of gamma-ray bursts with no afterglow, we can now say the theory was incorrect and our telescopes have not failed us,” Dr. Paul Hancock, from Curtin University and lead researcher of the paper published in The Astrophysical Journal, said in a statement.

The team stacked 200 separate observations on top of each other to re-create the image of a gamma-ray burst in better quality. However, they did not find a radio afterglow like expected.

“In our research paper we argue that there must be two distinct types of gamma-ray burst, likely linked to differences in the magnetic field of the exploding star,” Dr. Hancock said. “Gamma-ray bursts are thought to mark the birth of a Black Hole or Neutron Star – both of which have super-dense cores. But Neutron Stars have such strong magnetic fields (a million times stronger than those of Black Holes) that producing gamma-rays are more difficult.”

The researcher said they believe those stars that collapse to form a neutron star have energy left over to produce the radio afterglow, while those that become black holes exhaust their energy into that one final gamma-ray flash.

“We calculate that no more than 70 percent of GRB afterglows are truly radio bright, leaving a significant population of GRBs that lack a radio afterglow. These radio bright GRBs have higher gamma-ray fluence, isotropic energies, X-ray fluxes and optical fluxes than the radio faint GRBs, confirming the existence of two physically distinct populations,” the team wrote in the journal. “We suggest that the gamma-ray efficiency of the prompt emission is responsible for the difference between the two populations. We also discuss the implications for future radio and optical surveys.”

Scientists are now working to try and test out the team’s new theory and to determine if there are other ways in which the two types of bursts differ.

“We now have to take a whole new look at gamma-ray bursts – so far this work has shown that being wrong is sometimes more interesting than being right,” Dr. Hancock said.